Systems and methods for multimode wireless communication handoff

ABSTRACT

Methods and apparatus for autonomous handover between WiMAX (Worldwide Interoperability for Microwave Access) and CDMA (Code Division Multiple Access) EVDO (Evolution-Data Optimized) or 1×RTT (one times Radio Transmission Technology, or 1×) networks during normal operation of a dual-mode mobile station (MS) are provided. The methods and apparatus may improve service continuity during handover and need not require any changes to the WiMAX or CDMA standards.

CLAIM OF PRIORITY

This application claims the benefit of priority from, and is acontinuation-in-part of, U.S. patent application Ser. No. 12/176,304,filed Jul. 18, 2008 and entitled “Systems and Methods for MultimodeWireless Communication Handoff,” which claims the benefit of priorityfrom U.S. Provisional Patent Application Ser. No. 61/052,265, filed May11, 2008 and entitled “Systems and Methods for Multimode WirelessCommunication Handoff,” and from U.S. Provisional Patent ApplicationSer. No. 61/052,266, also filed May 11, 2008 and also entitled “Systemsand Methods for Multimode Wireless Communication Handoff,” all of whichare assigned to the assignee of this application and are fullyincorporated herein by reference for all purposes.

This application also claims the benefit of priority from U.S.Provisional Patent Application Ser. No. 61/052,259, filed May 11, 2008and entitled “Systems and Methods for Multimode Wireless CommunicationHandoff,” and from U.S. Provisional Patent Application Ser. No.61/052,260, also filed May 11, 2008 and also entitled “Systems andMethods for Multimode Wireless Communication Handoff,” both of which areassigned to the assignee of this application and are fully incorporatedherein by reference for all purposes.

TECHNICAL FIELD

Certain embodiments of the present disclosure generally relate towireless communications and, more particularly, to autonomous handoverof a mobile station (MS) from a WiMAX network to a CDMA, and vice versa.

BACKGROUND

Orthogonal frequency-division multiplexing (OFDM) and orthogonalfrequency division multiple access (OFDMA) wireless communicationsystems under IEEE 802.16 use a network of base stations to communicatewith wireless devices (i.e., mobile stations) registered for services inthe systems based on the orthogonality of frequencies of multiplesubcarriers and can be implemented to achieve a number of technicaladvantages for wideband wireless communications, such as resistance tomultipath fading and interference. Each base station (BS) emits andreceives radio frequency (RF) signals that convey data to and from themobile stations. For various reasons, such as a mobile station (MS)moving away from the area covered by one base station and entering thearea covered by another, a handover (also known as a handoff) may beperformed to transfer communication services (e.g., an ongoing call ordata session) from one base station to another.

Three handover methods are supported in IEEE 802.16e-2005: Hard Handoff(HHO), Fast Base Station Switching (FBSS) and Macro Diversity Handover(MDHO). Of these, supporting HHO is mandatory, while FBSS and MDHO aretwo optional alternatives.

HHO implies an abrupt transfer of connection from one BS to another. Thehandover decisions may be made by the MS or the BS based on measurementresults reported by the MS. The MS may periodically conduct an RF scanand measure the signal quality of neighboring base stations. Thehandover decision may arise, for example, from the signal strength fromone cell exceeding the current cell, the MS changing location leading tosignal fading or interference, or the MS requiring a higher Quality ofService (QoS). Scanning is performed during scanning intervals allocatedby the BS. During these intervals, the MS is also allowed to optionallyperform initial ranging and to associate with one or more neighboringbase stations. Once a handover decision is made, the MS may beginsynchronization with the downlink transmission of the target BS, mayperform ranging if it was not done while scanning, and may thenterminate the connection with the previous BS. Any undelivered ProtocolData Units (PDUs) at the BS may be retained until a timer expires.

When FBSS is supported, the MS and BS maintain a list of BSs that areinvolved in FBSS with the MS. This set is called a diversity set. InFBSS, the MS continuously monitors the base stations in the diversityset. Among the BSs in the diversity set, an anchor BS is defined. Whenoperating in FBSS, the MS only communicates with the anchor BS foruplink and downlink messages including management and trafficconnections. Transition from one anchor BS to another (i.e., BSswitching) can be performed if another BS in the diversity set hasbetter signal strength than the current anchor BS. Anchor updateprocedures are enabled by communicating with the serving BS via theChannel Quality Indicator Channel (CQICH) or the explicit handover (HO)signaling messages.

A FBSS handover begins with a decision by an MS to receive or transmitdata from the anchor BS that may change within the diversity set. The MSscans the neighbor BSs and selects those that are suitable to beincluded in the diversity set. The MS reports the selected BSs, and theBS and the MS update the diversity set. The MS may continuously monitorthe signal strength of the BSs that are in the diversity set and selectsone BS from the set to be the anchor BS. The MS reports the selectedanchor BS on CQICH or MS-initiated handover request message.

For MSs and BSs that support MDHO, the MS and BS maintain a diversityset of BSs that are involved in MDHO with the MS. Among the BSs in thediversity set, an anchor BS is defined. The regular mode of operationrefers to a particular case of MDHO with the diversity set consisting ofa single BS. When operating in MDHO, the MS communicates with all BSs inthe diversity set of uplink and downlink unicast messages and traffic.

An MDHO begins when an MS decides to transmit or receive unicastmessages and traffic from multiple BSs in the same time interval. Fordownlink MDHO, two or more BSs provide synchronized transmission of MSdownlink data such that diversity combining is performed at the MS. Foruplink MDHO, the transmission from an MS is received by multiple BSswhere selection diversity of the information received is performed.

SUMMARY

Certain embodiments of the present disclosure generally relate toperforming autonomous handover of a mobile station (MS) from one radioaccess technology (RAT) network to another different RAT network, suchas from a WiMAX network to a CDMA network, and vice versa. This handovermay occur during normal operation of an MS, thereby allowing betterservice continuity while the MS travels from one network to the next.

Certain embodiments of the present disclosure provide a method forperforming handover between network service via first and second RATs,wherein the first and second RATs are different. The method generallyincludes detecting a trigger to initiate a scan for network service viathe second RAT while communicating via the first RAT; initiating thescan for the second RAT, in response to detecting the trigger; anddetermining whether to handover to network service via the second RATbased on results of the scan.

Certain embodiments of the present disclosure provide a receiver forwireless communication. The receiver generally includestrigger-detection logic configured to detect a trigger to initiate ascan for network service via a second RAT while communicating via afirst RAT, wherein the first and second RATs are different;scan-initiation logic configured to initiate the scan for the secondRAT, in response to detecting the trigger; and handover-determinationlogic configured to determine whether to handover to network service viathe second RAT based on results of the scan.

Certain embodiments of the present disclosure provide an apparatus forperforming handover between network service via first and second RATs.The apparatus generally includes means for detecting a trigger toinitiate a scan for network service via the second RAT whilecommunicating via the first RAT, wherein the first and second RATs aredifferent; means for initiating the scan for the second RAT, in responseto detecting the trigger; and means for determining whether to handoverto network service via the second RAT based on results of the scan.

Certain embodiments of the present disclosure provide a mobile device.The mobile device generally includes a receiver front-end forcommunicating via a first RAT; trigger-detection logic configured todetect a trigger to initiate a scan for network service via the secondRAT while communicating via the first RAT, wherein the first and secondRATs are different; scan-initiation logic configured to initiate thescan for the second RAT, in response to detecting the trigger; andhandover-determination logic configured to determine whether to handoverto network service via the second RAT based on results of the scan.

Certain embodiments of the present disclosure provide acomputer-readable medium containing a program for performing handoverbetween network service via first and second radio RATs, which, whenexecuted by a processor, performs certain operations. The operationsgenerally include detecting a trigger to initiate a scan for networkservice via the second RAT while communicating via the first RAT;initiating the scan for the second RAT, in response to detecting thetrigger; and determining whether to handover to network service via thesecond RAT based on results of the scan.

Certain embodiments of the present disclosure provide a computer-programapparatus for performing handover between network service via first andsecond radio access technologies (RATs) comprising a computer readablemedium having instructions stored thereon. The instructions areexecutable by one or more processors. The instructions includeinstructions for, while communicating via the first RAT, detecting atrigger to initiate a scan for network service via the second RAT,wherein the first and second RATs are different. The instructionsinclude instructions for initiating the scan for the second RAT, inresponse to detecting the trigger. The instructions include instructionsfor determining whether to handover to network service via the secondRAT based on results of the scan.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentdisclosure can be understood in detail, a more particular description,briefly summarized above, may be had by reference to embodiments, someof which are illustrated in the appended drawings. It is to be noted,however, that the appended drawings illustrate only certain typicalembodiments of this disclosure and are therefore not to be consideredlimiting of its scope, for the description may admit to other equallyeffective embodiments.

FIG. 1 illustrates an example wireless communication system, inaccordance with certain embodiments of the present disclosure.

FIG. 2 illustrates various components that may be utilized in a wirelessdevice, in accordance with certain embodiments of the presentdisclosure.

FIG. 3 illustrates an example transmitter and an example receiver thatmay be used within a wireless communication system that utilizesorthogonal frequency-division multiplexing and orthogonal frequencydivision multiple access (OFDM/OFDMA) technology, in accordance withcertain embodiments of the present disclosure.

FIG. 4A illustrates a mobility scenario where a dual-mode mobile station(MS) may move outside the coverage of a WiMAX radio access network andenter the coverage of a CDMA EVDO/1× network, in accordance with certainembodiments of the present disclosure.

FIG. 4B illustrates a mobility scenario where a dual-mode MS may moveoutside the coverage of a CDMA EVDO radio access network and enter thecoverage of a WiMAX network, in accordance with certain embodiments ofthe present disclosure.

FIG. 5 is a flow chart of example operations for performing anautonomous handover of a dual-mode MS from a WiMAX network to a CDMAEVDO/1× network, in accordance with certain embodiments of the presentdisclosure.

FIG. 5A is a block diagram of means corresponding to the exampleoperations of FIG. 5 for performing an autonomous handover from a WiMAXnetwork to a CDMA EVDO/1× network, in accordance with certainembodiments of the present disclosure.

FIG. 6 illustrates example CDMA scanning intervals requested by an MScommunicating using a WiMAX network service during the interleavingintervals, in accordance with certain embodiments of the presentdisclosure.

FIG. 7 illustrates a call flow of example operations for performing anMS-autonomous handover from a WiMAX base station to a CDMA EVDO/1× basestation, in accordance with certain embodiments of the presentdisclosure.

FIG. 8 is a flow chart of example operations for performing anautonomous handover of a dual-mode MS from a CDMA EVDO network to aWiMAX network, in accordance with certain embodiments of the presentdisclosure.

FIG. 8A is a block diagram of means corresponding to the exampleoperations of FIG. 8 for performing an autonomous handover from a CDMAEVDO network to a WiMAX network, in accordance with certain embodimentsof the present disclosure.

FIG. 9 illustrates a call flow of example operations for performing anMS-autonomous handover from a CDMA EVDO base station to a WiMAX basestation, in accordance with certain embodiments of the presentdisclosure.

DETAILED DESCRIPTION

Certain embodiments of the present disclosure provide methods andapparatus for autonomous handover between WiMAX and CDMA EVDO/1×networks during normal operation of a dual-mode mobile station (MS). Themethods and apparatus may improve service continuity during handover andneed not require any changes to the WiMAX or the CDMA standards.

Exemplary Wireless Communication System

The methods and apparatus of the present disclosure may be utilized in abroadband wireless communication system. The term “broadband wireless”refers to technology that provides wireless, voice, Internet, and/ordata network access over a given area.

WiMAX, which stands for the Worldwide Interoperability for MicrowaveAccess, is a standards-based broadband wireless technology that provideshigh-throughput broadband connections over long distances. There are twomain applications of WiMAX today: fixed WiMAX and mobile WiMAX. FixedWiMAX applications are point-to-multipoint, enabling broadband access tohomes and businesses, for example. Mobile WiMAX offers the full mobilityof cellular networks at broadband speeds.

Mobile WiMAX is based on OFDM (orthogonal frequency-divisionmultiplexing) and OFDMA (orthogonal frequency division multiple access)technology. OFDM is a digital multi-carrier modulation technique thathas recently found wide adoption in a variety of high-data-ratecommunication systems. With OFDM, a transmit bit stream is divided intomultiple lower-rate substreams. Each substream is modulated with one ofmultiple orthogonal subcarriers and sent over one of a plurality ofparallel subchannels. OFDMA is a multiple access technique in whichusers are assigned subcarriers in different time slots. OFDMA is aflexible multiple-access technique that can accommodate many users withwidely varying applications, data rates, and quality of servicerequirements.

The rapid growth in wireless internets and communications has led to anincreasing demand for high data rate in the field of wirelesscommunications services. OFDM/OFDMA systems are today regarded as one ofthe most promising research areas and as a key technology for the nextgeneration of wireless communications. This is due to the fact thatOFDM/OFDMA modulation schemes can provide many advantages such asmodulation efficiency, spectrum efficiency, flexibility, and strongmultipath immunity over conventional single carrier modulation schemes.

IEEE 802.16x is an emerging standard organization to define an airinterface for fixed and mobile broadband wireless access (BWA) systems.IEEE 802.16x approved “IEEE P802.16-REVd/D5-2004” in May 2004 for fixedBWA systems and published “IEEE P802.16e/D12 October 2005” in October2005 for mobile BWA systems. Those two standards defined four differentphysical layers (PHYs) and one media access control (MAC) layer. TheOFDM and OFDMA physical layer of the four physical layers are the mostpopular in the fixed and mobile BWA areas respectively.

FIG. 1 illustrates an example of a wireless communication system 100.The wireless communication system 100 may be a broadband wirelesscommunication system. The wireless communication system 100 may providecommunication for a number of cells 102, each of which is serviced by abase station 104. A base station 104 may be a fixed station thatcommunicates with user terminals 106. The base station 104 mayalternatively be referred to as an access point, a Node B, or some otherterminology.

FIG. 1 depicts various user terminals 106 dispersed throughout thesystem 100. The user terminals 106 may be fixed (i.e., stationary) ormobile. The user terminals 106 may alternatively be referred to asremote stations, access terminals, terminals, subscriber units, mobilestations, stations, user equipment, etc. The user terminals 106 may bewireless devices, such as cellular phones, personal digital assistants(PDAs), handheld devices, wireless modems, laptop computers, personalcomputers (PCs), etc.

A variety of algorithms and methods may be used for transmissions in thewireless communication system 100 between the base stations 104 and theuser terminals 106. For example, signals may be sent and receivedbetween the base stations 104 and the user terminals 106 in accordancewith OFDM/OFDMA techniques. If this is the case, the wirelesscommunication system 100 may be referred to as an OFDM/OFDMA system.

A communication link that facilitates transmission from a base station104 to a user terminal 106 may be referred to as a downlink 108, and acommunication link that facilitates transmission from a user terminal106 to a base station 104 may be referred to as an uplink 110.Alternatively, a downlink 108 may be referred to as a forward link or aforward channel, and an uplink 110 may be referred to as a reverse linkor a reverse channel.

A cell 102 may be divided into multiple sectors 112. A sector 112 is aphysical coverage area within a cell 102. Base stations 104 within awireless communication system 100 may utilize antennas that concentratethe flow of power within a particular sector 112 of the cell 102. Suchantennas may be referred to as directional antennas.

FIG. 2 illustrates various components that may be utilized in a wirelessdevice 202. The wireless device 202 is an example of a device that maybe configured to implement the various methods described herein. Thewireless device 202 may be a base station 104 or a user terminal 106.

The wireless device 202 may include a processor 204 which controlsoperation of the wireless device 202. The processor 204 may also bereferred to as a central processing unit (CPU). Memory 206, which mayinclude both read-only memory (ROM) and random access memory (RAM),provides instructions and data to the processor 204. A portion of thememory 206 may also include non-volatile random access memory (NVRAM).The processor 204 typically performs logical and arithmetic operationsbased on program instructions stored within the memory 206. Theinstructions in the memory 206 may be executable to implement themethods described herein.

The wireless device 202 may also include a housing 208 that may includea transmitter 210 and a receiver 212 to allow transmission and receptionof data between the wireless device 202 and a remote location. Thetransmitter 210 and receiver 212 may be combined into a transceiver 214.An antenna 216 may be attached to the housing 208 and electricallycoupled to the transceiver 214. The wireless device 202 may also include(not shown) multiple transmitters, multiple receivers, multipletransceivers, and/or multiple antennas.

The wireless device 202 may also include a signal detector 218 that maybe used in an effort to detect and quantify the level of signalsreceived by the transceiver 214. The signal detector 218 may detect suchsignals as total energy, pilot energy from pilot subcarriers or signalenergy from the preamble symbol, power spectral density, and othersignals. The wireless device 202 may also include a digital signalprocessor (DSP) 220 for use in processing signals.

The various components of the wireless device 202 may be coupledtogether by a bus system 222, which may include a power bus, a controlsignal bus, and a status signal bus in addition to a data bus.

FIG. 3 illustrates an example of a transmitter 302 that may be usedwithin a wireless communication system 100 that utilizes OFDM/OFDMA.Portions of the transmitter 302 may be implemented in the transmitter210 of a wireless device 202. The transmitter 302 may be implemented ina base station 104 for transmitting data 306 to a user terminal 106 on adownlink 108. The transmitter 302 may also be implemented in a userterminal 106 for transmitting data 306 to a base station 104 on anuplink 110.

Data 306 to be transmitted is shown being provided as input to aserial-to-parallel (S/P) converter 308. The S/P converter 308 may splitthe transmission data into N parallel data streams 310.

The N parallel data streams 310 may then be provided as input to amapper 312. The mapper 312 may map the N parallel data streams 310 ontoN constellation points. The mapping may be done using some modulationconstellation, such as binary phase-shift keying (BPSK), quadraturephase-shift keying (QPSK), 8 phase-shift keying (8PSK), quadratureamplitude modulation (QAM), etc. Thus, the mapper 312 may output Nparallel symbol streams 316, each symbol stream 316 corresponding to oneof the N orthogonal subcarriers of the inverse fast Fourier transform(IFFT) 320. These N parallel symbol streams 316 are represented in thefrequency domain and may be converted into N parallel time domain samplestreams 318 by an IFFT component 320.

A brief note about terminology will now be provided. N parallelmodulations in the frequency domain are equal to N modulation symbols inthe frequency domain, which are equal to N mapping and N-point IFFT inthe frequency domain, which is equal to one (useful) OFDM symbol in thetime domain, which is equal to N samples in the time domain. One OFDMsymbol in the time domain, N_(s), is equal to N_(cp) (the number ofguard samples per OFDM symbol)+N (the number of useful samples per OFDMsymbol).

The N parallel time domain sample streams 318 may be converted into anOFDM/OFDMA symbol stream 322 by a parallel-to-serial (P/S) converter324. A guard insertion component 326 may insert a guard interval betweensuccessive OFDM/OFDMA symbols in the OFDM/OFDMA symbol stream 322. Theoutput of the guard insertion component 326 may then be upconverted to adesired transmit frequency band by a radio frequency (RF) front end 328.An antenna 330 may then transmit the resulting signal 332.

FIG. 3 also illustrates an example of a receiver 304 that may be usedwithin a wireless communication system 100 that utilizes OFDM/OFDMA.Portions of the receiver 304 may be implemented in the receiver 212 of awireless device 202. The receiver 304 may be implemented in a userterminal 106 for receiving data 306 from a base station 104 on adownlink 108. The receiver 304 may also be implemented in a base station104 for receiving data 306 from a user terminal 106 on an uplink 110.

The transmitted signal 332 is shown traveling over a wireless channel334. When a signal 332′ is received by an antenna 330′, the receivedsignal 332′ may be downconverted to a baseband signal by an RF front end328′. A guard removal component 326′ may then remove the guard intervalthat was inserted between OFDM/OFDMA symbols by the guard insertioncomponent 326.

The output of the guard removal component 326′ may be provided to an S/Pconverter 324′. The S/P converter 324′ may divide the OFDM/OFDMA symbolstream 322′ into the N parallel time-domain symbol streams 318′, each ofwhich corresponds to one of the N orthogonal subcarriers. A fast Fouriertransform (FFT) component 320′ may convert the N parallel time-domainsymbol streams 318′ into the frequency domain and output N parallelfrequency-domain symbol streams 316′.

A demapper 312′ may perform the inverse of the symbol mapping operationthat was performed by the mapper 312, thereby outputting N parallel datastreams 310′. A P/S converter 308′ may combine the N parallel datastreams 310′ into a single data stream 306′. Ideally, this data stream306′ corresponds to the data 306 that was provided as input to thetransmitter 302.

Exemplary Handover from WiMAX to CDMA

FIG. 4A illustrates a mobility scenario where WiMAX cells 102 areadjacent to Code Division Multiple Access (CDMA) cells 404. At leastsome of the WiMAX cells 102 may also provide coverage for CDMA signals,but for purposes of certain embodiments in the present disclosure, thecells 102 currently utilize WiMAX for communicating with the MS 420.Each WiMAX cell 102 typically has a WiMAX base station (BS) 104 tofacilitate WiMAX network communications with a user terminal, such as adual-mode MS 420. As used herein, a dual-mode MS generally refers to anMS that is capable of processing both WiMAX and CDMA signals. Similar toa WiMAX cell 102, each CDMA cell 404 typically has a CDMA BS 410 inorder to facilitate CDMA Evolution-Data Optimized (EVDO) or 1 timesRadio Transmission Technology (1×RTT, or simply 1×) communications, forexample, with a user terminal, such as the MS 420.

In the present scenario of FIG. 4A, the MS 420 may move outside thecoverage area of a WiMAX BS 104 and enter the coverage area of a CDMA BS410. While transitioning from a WiMAX cell 102 to a CDMA cell 404 asshown, the MS 420 may enter a coverage overlap area 408 where the MS isable to receive signals from both networks.

It is during this transition that the MS may implement a handoverprocess from a WiMAX BS to a CDMA BS. In addition to the normaldifficulties associated with handover between two BSs of the samenetwork type, handover between two BSs of different network types, suchas from WiMAX to CDMA EVDO/1×, presents further challenges to servicecontinuity, which are particularly acute if the MS is in the process ofdata transfer when the handover occurs.

Therefore, there is a need for techniques and apparatus such that adual-mode MS may quickly and autonomously perform a handover from theWiMAX network to the CDMA network while minimizing service disruption.

Embodiments of the present disclosure provide methods and apparatusallowing a dual-mode MS to handover from a WiMAX network to a CDMAEVDO/1× network. Such techniques may increase service continuity whilethe MS moves from WiMAX to CDMA network coverage. Furthermore,embodiments of the present disclosure do not require any standardchanges, and the handover may be automatically performed by the MS(i.e., the handover is an MS-autonomous procedure).

FIG. 5 shows a flowchart of example operations for MS-autonomoushandover from a WiMAX network to a CDMA EVDO/1× network. The operationsbegin, at 500, by detecting a trigger which may cause a dual-mode MS toscan for possible CDMA coverage. A trigger event may be intentionallyperiodic, may occur depending on the supported or selected handovermethod such as HHO, FBSS, or MDHO, or may occur when the number ofneighbor BSs received in the Neighbor Advertisement Message is less thana number received in the past, for example.

Periodic triggering may occur at certain time intervals regardless ofthe status of the MS. For some embodiments, these time intervals may bepreset in the MS, and may be subsequently updated with new timeintervals if desired.

When an MS supports HHO, the trigger event may occur when the meancarrier-to-interference-plus-noise ratio (CINR) or mean received signalstrength indicator (RSSI) of the serving WiMAX BS falls below a firstthreshold, and there is no neighbor WiMAX BS with a mean CINR or meanRSSI more than a second threshold, wherein the first and secondthresholds may differ. For example, the serving WiMAX threshold andneighbor WiMAX threshold may be respectively represented asT_ScanCDMA_(—)1 and T_ScanCDMA_(—)2. In essence, this trigger event mayoccur when the MS has currently moved beyond the effective coverage ofthe serving WiMAX BS and there is no appropriate WiMAX BS to which tohandover.

For an MS that supports FBSS or MDHO, triggering may occur when the meanCINR of all WiMAX BSs in a diversity set falls below a certainthreshold. For example, this threshold may be represented asT_ScanCDMA_(—)3, and furthermore may be equal to (1+γ)*H_Delete, whereinγ is an adjustment factor ≧0 and H_Delete is a threshold used byFBSS/MDHO-capable MSs to determine when to drop a BS from the diversityset. With the adjustment factor γ, the triggering threshold may begreater than the H_Delete threshold in an effort to trigger scanning forCDMA coverage sufficiently before the moving MS has lost WiMAX coverageand tries to initiate a handover. In sum, this trigger event may occurwhen all nearby WiMAX BS CINR values experience a drop below a certainthreshold, therefore predicting that the MS is going to move orindicating that the MS has moved outside the effective coverage of theWiMAX network.

Neighbor Advertisement Message triggering may occur when the number ofneighboring WiMAX BSs received in the Neighbor Advertisement Message(MOB_NBR-ADV) is less than β multiplied with the average number of theneighbor BSs in the MOB_NBR-ADV message received in the past, wherein βis an adjustment factor ≧0. For example, scanning may be triggered uponreceiving the n^(th) MOB_NBR-ADV message in whichN_NBR(n)<β*A_N_NBR(n−1), n=0, 1, 2 . . . , where N_NBR(n) is the numberof neighbor BSs in the current MOB_NBR-ADV message,A_N_NBR(n)=α*N_NBR(n)+(1−α)*A_N_NBR(n−1) is the exponential movingaverage, and α is the smoothing factor for the moving average.

Because the WiMAX BS may continue to broadcast the same neighbormessage, the index n need not be incremented—and the average A_N_NBR(n)need not be calculated—after receiving each MOB_NBR-ADV message. Rather,the index n may be incremented in the event of a handover or an updateto the anchor BS in MDHO or FBSS when the MS may receive a differentNeighbor Advertisement Message.

In order to scan the CDMA EVDO/1× network without losing data packets inthe WiMAX network, any current data transmissions may be temporarilysuspended. Thus, when one of the above trigger conditions is met, the MSmay request suspension of any current data transmission with the WiMAXnetwork by sending a Scanning Interval Allocation Request (MOB_SCN-REQ)message at 510 to the WiMAX BS in an effort to notify the BS of certaintime intervals when the MS may be unavailable for communication with theWiMAX network to scan the CDMA EVDO/1× network.

The MOB_SCN-REQ message may comprise parameters such as scan duration,interleaving interval, and scan iteration. The scan duration may be theduration (in units of OFDM/OFDMA frames) of the requested scanningperiod, the interleaving interval may be the period of MS normaloperations interleaved between scanning durations, and the scaniteration may be the requested number of iterating scanning interval(s)by an MS. These parameters are discussed in greater detail below withrespect to FIG. 6.

After receiving the scanning request message, the WiMAX BS may thenrespond with a Scanning Interval Allocation Response (MOB_SCN-RSP)message. The BS may either grant or deny the scanning request.

Upon triggering a scan for the CDMA EVDO/1× network, the MS may scan theCDMA network at 520 using a preferred roaming list (PRL), which may bepre-programmed in the MS. The PRL may provide CDMA channel informationin an effort to scan for possible CDMA pilots, synchronize to the CDMAnetwork, and/or acquire the Sector Parameter Message or System ParameterMessage. All CDMA BSs successfully identified in scanning may beincluded in the CDMA pilot candidate set. Each candidate pilot maycomprise the following attributes: EVDO or 1× protocol revision; BandClass; Channel Number; System Identification Number (SID), NetworkIdentification Number (NID), Packet Zone ID, and Pilot Pseudo Noise (PN)Offset.

FIG. 6 shows the scanning intervals in which the MS performs the CDMA BSscan. Upon detecting a trigger for a CDMA EVDO/1× network scan at 500,the MS may begin scanning for networks, shown by the Start Frame 610.Thereafter, the MS may scan for CDMA networks for a predetermined scanduration 620 at the end of which, the MS may discontinue the scan for apredetermined interleaving interval 622 and resume normal operation.This alternating pattern of scanning and interleaving may continue untilthe end of the CDMA BS scan. Rather than multiple scan iterations, theMOB_SCN-REQ scan iteration parameter may indicate a single scaniteration for some embodiments. In such cases, the scan for CDMA BSs mayonly include a single scan duration.

Each time scanning completes, one or more new candidate CDMA pilot(s)may be added in the candidate set. Conversely, one or more existingcandidate CDMA pilot(s) may be deleted from the CDMA candidate set ifthe pilot is no longer found during scanning.

Depending on the results of the CDMA BS scan, the MS may autonomouslydetermine whether to initiate a handover to the CDMA BS at 530. Thedecision to handover may be triggered depending on the handover methodsupported by the MS, in addition to the CDMA BS scan indicating thatsome candidate CDMA BS is available. For HHO, the handover may betriggered when the serving BS has a mean CINR less than a thresholdand/or mean RSSI less than another threshold and/or BS Round Trip Delay(RTD) more than yet another threshold. For FBSS or MDHO, handover may betriggered when all BSs in the diversity set are about to drop, namelywith mean CINR less than H_Delete.

If the decision to handover to a CDMA BS is made at 530, then duringhandover at 540, the MS may signal intent to enter an idle state bysending a De-register Request (DREG-REQ) message to the serving WiMAXBS. Upon receiving a response from the WiMAX BS (e.g., a De-registerCommand (DREG-CMD) message) or a timeout, the MS may terminateconnection with the WiMAX BS. After terminating the data connection, theMS may search all CDMA pilots in the candidate set and measure the pilotstrength of each pilot. Then the MS may choose the strongest pilot foraccess to the CDMA EVDO/1× network. The MS may then start access and setup a new data session and connection with the CDMA BS associated withthe strongest pilot.

However, if no pilots are found in the candidate set, the MS may begin afresh CDMA channel search to identify possible CDMA pilots for access.Additionally, if the handover to the CDMA EVDO/1× network fails before apredetermined deadline, the MS may still return to the WiMAX networkusing the network reentry after idle mode procedure as specified in theWiMAX standards to resume the previous data session.

FIG. 7 further illustrates the MS-autonomous WiMAX to CDMA EVDO/1×handover procedure and details the interaction between the dual-mode MS420, the WiMAX BS 104, and the CDMA BS 410. As stated previously, theWiMAX to CDMA EVDO/1× handover process may begin with a trigger forscanning the CDMA network at 730. The MS may then send a scanningrequest (MOB_SCN-REQ) to the WiMAX BS at 740. At 750, the WiMAX BS mayrespond with a scanning response (MOB_SCN-RSP) granting the request.Thereafter, the MS may scan the CDMA EVDO/1× BSs and include allpossible CDMA pilots in the candidate set at 760. When a trigger foractual handover is received at 770, the MS may send a De-registerRequest (DREG-REQ) to the WiMAX BS at 780. In response at 785, the WiMAXBS may send a De-register Command (DREG-CMD) to instruct the MS toterminate normal operations with the WiMAX BS. The MS may then accessthe new CDMA EVDO/1× BS and set up a new data session and connection at790.

Exemplary Handover from CDMA to WiMAX

FIG. 4B illustrates a mobility scenario where Code Division MultipleAccess (CDMA) cells 404 are adjacent to WiMAX cells 102. At least someof the CDMA cells 404 may also provide coverage for WiMAX signals, butfor purposes of certain embodiments in the present disclosure, the CDMAcells 404 may currently utilize CDMA Evolution-Data Optimized (EVDO) forcommunicating with a user terminal, such as a dual-mode MS 420. EachCDMA cell 404 typically has a CDMA BS 410 to facilitate CDMA EVDOnetwork communications with the dual-mode MS 420.

In the present scenario of FIG. 4B, the MS 420 may move outside thecoverage area of a CDMA BS 410 and enter the coverage area of a WiMAX BS104. While transitioning from a CDMA cell 404 to a WiMAX cell 102 asshown, the MS 420 may enter a coverage overlap area 408 where the MS isable to receive signal from both networks.

It is during this transition that the MS may implement a handoverprocess from a CDMA BS to a WiMAX BS. In addition to the normaldifficulties associated with handover between two BSs of the samenetwork type, handover between two BSs of different network types, suchas from CDMA EVDO to WiMAX, presents further challenges to servicecontinuity, which are particularly acute if the MS is in the process ofdata transfer when the handover occurs.

Therefore, there is a need for techniques and apparatus such that adual-mode MS may quickly and autonomously perform a handover from a CDMAnetwork to a WiMAX network while minimizing service disruption.

Embodiments of the present disclosure provide methods and apparatusallowing a dual-mode MS to handover from a CDMA EVDO network to a WiMAXnetwork. Such techniques may increase service continuity while the MSmoves from CDMA to WiMAX network coverage. Furthermore, the embodimentsof the present disclosure do not require any standard changes, and thehandover may be automatically performed by the MS (i.e., the handover isan MS-autonomous procedure).

FIG. 8 shows a flowchart of example operations for MS-autonomoushandover from a CDMA EVDO network to a WiMAX network. The operationsbegin, at 800, by detecting a trigger which may cause a dual-mode MS toscan for possible WiMAX coverage. A trigger event may be intentionallyperiodic, may occur depending on the pilot strength threshold in theCDMA active set, or may occur depending on a number-of-neighborsthreshold, for example.

Periodic triggering may occur at certain time intervals regardless ofthe status of the MS. For some embodiments, these time intervals may bepreset in the MS, and may be subsequently updated with new timeintervals if desired.

Pilot strength threshold triggering may occur when all the pilots in theCDMA active set have a pilot strength less than a certain threshold. Forexample, this threshold may be represented as T_ScanWiMAX, which may berepresented as (1+γ)*T_DROP, wherein γ is an adjustment factor ≧0 andT_DROP is a threshold used by MSs to determine when to drop a pilot fromthe CDMA active set. With the adjustment factor γ, the triggeringthreshold may be greater than the T_DROP threshold in an effort totrigger scanning for WiMAX coverage sufficiently before the moving MShas lost CDMA coverage and tries to initiate a handover. In sum, thistrigger event may occur when all nearby CDMA BS pilot strength valuesdrop below a certain threshold, therefore predicting that the MS isgoing to move or indicating that the MS has moved outside the effectivecoverage area of the CDMA network.

Number-of-neighbors threshold triggering may occur when the number ofneighboring CDMA BSs received in the Neighbor List Message, ExtendedNeighbor List Message, General Neighbor List Message, or UniversalNeighbor List Message is less than β multiplied with the average numberof the neighbors in the (Extended/General/Universal) Neighbor ListMessages received in the past, wherein β is an adjustment factor ≧0. Forexample, scanning may be triggered upon receiving the n_(th)(Extended/General/Universal) Neighbor List Message in whichN_NBR(n)<β*A_N_NBR(n−1), n=0, 1, 2 . . . , where N_NBR(n) is the numberof neighbor sectors in the current (Extended/General/Universal) NeighborList Message, A_N_NBR(n)=α*N_NBR(n)+(1−α)*A_N_NBR(n−1) is theexponential moving average, and α is the smoothing factor.

Upon triggering a scan for the WiMAX network, the MS may initiate aWiMAX network scan at 810. In order to scan the WiMAX network withoutlosing data packets in the CDMA EVDO network, any current datatransmissions may be temporarily suspended. Thus, when one of the abovetrigger conditions is met, the MS may request suspension of any currentdata transmission with the CDMA EVDO network by sending “null cover” asthe Data Rate Control (DRC) cover to the CDMA BS in an effort to notifythe BS of certain time intervals when the MS may be unavailable forcommunication with the CDMA EVDO network to scan the WiMAX network.

After sending a DRC cover to the CDMA EVDO BS, the MS may scan the WiMAXnetwork at 820 using assistance information, which may be pre-programmedinto the MS. For example, the assistance information may comprise: bandclass, bandwidth, FFT size, and ratio of cyclic prefix (CP). Using thisinformation, the MS may search for the WiMAX BS preamble, synchronize tothe WiMAX framing, read the DL-MAP, or even acquire the Downlink ChannelDescriptor (DCD) and the Uplink Channel Descriptor (UCD) messages.Thereafter, WiMAX BSs in the neighboring area that are successfullyidentified through scanning may be added into the WiMAX BS candidateset. Each candidate WiMAX BS in the candidate set may include thefollowing attributes: BS ID, bandwidth, FFT size, ratio of CP, FrequencyAssignment (FA) index, frame size, preamble index, and optional DCD/UCD.

Following the scan, the MS may notify the CDMA EVDO BS of completion ofthe scanning process by sending a DRC Cover=Sector Cover message to theCDMA EVDO BS. Additionally, one or more new candidate WiMAX BS(s) may beadded into the candidate set. Conversely, one or more existing candidateWiMAX BS(s) may be deleted from the candidate set if the candidate WiMAXBS(s) is/are no longer found during scanning.

Depending on the results of the WiMAX BS scan, the MS may autonomouslydetermine whether to initiate a handover to the WiMAX BS at 830. Thedecision to handover may depend on which trigger event occurred, inaddition to the WiMAX BS scan indicating that some candidate WiMAX BS isavailable. For example, handover may occur when all the pilots in theactive set are about to be dropped.

If the decision to handover to a WiMAX BS is made at 830, then duringhandover at 840, the MS may send a Connection Close message to the CDMABS in an effort to close the data connection with the CDMA EVDO networkand enter a dormant state. Next, the MS may scan all the WiMAX BSs inthe candidate set and measure the channel quality according to the CINRand/or the RSSI. The MS may choose the most proper WiMAX BS candidatewith either the largest CINR or RSSI, for example, for access to theWiMAX network. The MS may then initiate network entry access and set upa new data session and connection with the selected WiMAX BS.

However, if no WiMAX BSs are found in the candidate set, the MS maybegin a fresh WiMAX channel search in order to identify possible WiMAXBSs for access. Additionally, if the handover to the WiMAX network failsbefore a predetermined deadline, the MS may still return to the CDMAEVDO network using the reactivation from dormancy procedure specified inthe CDMA EVDO standards to resume the previous data session.

FIG. 9 further illustrates the MS-autonomous CDMA EVDO to WiMAX handoverprocedure and details the interaction between the dual-mode MS 420, theCDMA BS 410, and the WiMAX BS 104. The handover process may begin with atrigger for scanning the WiMAX BSs at 930. The MS may then notify theCDMA EVDO BS of the impending scan by sending a DRC Cover=Null Covermessage at 940. Thereafter, the MS may scan for WiMAX BSs and includeall possible WiMAX BSs in the candidate set at 950. Following the scan,the MS may notify the CDMA EVDO BS of completion of the scanning processby sending a DRC Cover=Sector Cover message to the CDMA EVDO BS at 960.Upon triggering a handover at 970, the MS may send a Connection Closemessage at 980 to the CDMA EVDO BS whereupon the MS may measure thechannel quality of the WiMAX BSs in the current candidate set, choosethe handover target, perform initial network entry to the target WiMAXBS, and set up the new data session and connection at 990.

The various operations of methods described above may be performed byvarious hardware and/or software component(s) and/or module(s)corresponding to means-plus-function blocks illustrated in the Figures.Generally, where there are methods illustrated in Figures havingcorresponding counterpart means-plus-function Figures, the operationblocks correspond to means-plus-function blocks with similar numbering.For example, blocks 500-540 illustrated in FIG. 5 correspond tomeans-plus-function blocks 500A-540A illustrated in FIG. 5A, and blocks800-840 illustrated in FIG. 8 correspond to means-plus-function blocks800A-840A illustrated in FIG. 8A.

As used herein, the term “determining” encompasses a wide variety ofactions. For example, “determining” may include calculating, computing,processing, deriving, investigating, looking up (e.g., looking up in atable, a database or another data structure), ascertaining and the like.Also, “determining” may include receiving (e.g., receiving information),accessing (e.g., accessing data in a memory) and the like. Also,“determining” may include resolving, selecting, choosing, establishingand the like.

Information and signals may be represented using any of a variety ofdifferent technologies and techniques. For example, data, instructions,commands, information, signals and the like that may be referencedthroughout the above description may be represented by voltages,currents, electromagnetic waves, magnetic fields or particles, opticalfields or particles or any combination thereof.

The techniques described herein may be used for various communicationsystems, including communication systems that are based on an orthogonalmultiplexing scheme. Examples of such communication systems includeOrthogonal Frequency Division Multiple Access (OFDMA) systems,Single-Carrier Frequency Division Multiple Access (SC-FDMA) systems, andso forth. An OFDMA system utilizes orthogonal frequency divisionmultiplexing (OFDM), which is a modulation technique that partitions theoverall system bandwidth into multiple orthogonal sub-carriers. Thesesub-carriers may also be called tones, bins, etc. With OFDM, eachsub-carrier may be independently modulated with data. An SC-FDMA systemmay utilize interleaved FDMA (IFDMA) to transmit on sub-carriers thatare distributed across the system bandwidth, localized FDMA (LFDMA) totransmit on a block of adjacent sub-carriers, or enhanced FDMA (EFDMA)to transmit on multiple blocks of adjacent sub-carriers. In general,modulation symbols are sent in the frequency domain with OFDM and in thetime domain with SC-FDMA.

The various illustrative logical blocks, modules and circuits describedin connection with the present disclosure may be implemented orperformed with a general purpose processor, a digital signal processor(DSP), an application specific integrated circuit (ASIC), a fieldprogrammable gate array signal (FPGA) or other programmable logic device(PLD), discrete gate or transistor logic, discrete hardware componentsor any combination thereof designed to perform the functions describedherein. A general purpose processor may be a microprocessor, but in thealternative, the processor may be any commercially available processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thepresent disclosure may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside (e.g., stored, encoded, etc.) in any form ofstorage medium that is known in the art. Some examples of storage mediathat may be used include random access memory (RAM), read only memory(ROM), flash memory, EPROM memory, EEPROM memory, registers, a harddisk, a removable disk, a CD-ROM and so forth. A software module maycomprise a single instruction, or many instructions, and may bedistributed over several different code segments, among differentprograms, and across multiple storage media. A storage medium may becoupled to a processor such that the processor can read informationfrom, and write information to, the storage medium. In the alternative,the storage medium may be integral to the processor.

The methods disclosed herein comprise one or more steps or actions forachieving the described method. The method steps and/or actions may beinterchanged with one another without departing from the scope of theclaims. In other words, unless a specific order of steps or actions isspecified, the order and/or use of specific steps and/or actions may bemodified without departing from the scope of the claims.

The functions described may be implemented in hardware, software,firmware, or any combination thereof. If implemented in software, thefunctions may be stored as instructions or as one or more sets ofinstructions on a computer-readable medium or storage medium. A storagemedia may be any available media that can be accessed by a computer orby one or more processing devices. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code in the form of instructions or datastructures and that can be accessed by a computer. Disk and disc, asused herein, include compact disc (CD), laser disc, optical disc,digital versatile disc (DVD), floppy disk, and Blu-ray® disc where disksusually reproduce data magnetically, while discs reproduce dataoptically with lasers.

Software or instructions may also be transmitted over a transmissionmedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition oftransmission medium.

Further, it should be appreciated that modules and/or other appropriatemeans for performing the methods and techniques described herein can bedownloaded and/or otherwise obtained by a user terminal and/or basestation as applicable. For example, such a device can be coupled to aserver to facilitate the transfer of means for performing the methodsdescribed herein. Alternatively, various methods described herein can beprovided via storage means (e.g., RAM, ROM, a physical storage mediumsuch as a compact disc (CD) or floppy disk, etc.), such that a userterminal and/or base station can obtain the various methods uponcoupling or providing the storage means to the device. Moreover, anyother suitable technique for providing the methods and techniquesdescribed herein to a device can be utilized.

It is to be understood that the claims are not limited to the preciseconfiguration and components illustrated above. Various modifications,changes and variations may be made in the arrangement, operation anddetails of the methods and apparatus described above without departingfrom the scope of the claims.

What is claimed is:
 1. A method for performing handover between networkservice via first and second radio access technologies (RATs),comprising: while communicating via the first RAT, detecting a triggerto initiate a scan for network service via the second RAT, wherein thefirst and second RATs are different, and wherein the trigger is detectedwhen a number of neighbors received in a neighbor-listing message isless than a number of neighbors previously received in aneighbor-listing message; initiating the scan for the second RAT, inresponse to detecting the trigger; and determining whether to handoverto network service via the second RAT based on results of the scan. 2.The method of claim 1, wherein detecting the trigger comprises reactingto a periodic trigger.
 3. The method of claim 1, wherein the first RATis WiMAX (Worldwide Interoperability for Microwave Access) and thesecond RAT is CDMA (Code Division Multiple Access) EVDO (Evolution-DataOptimized) or 1×RTT (one times Radio Transmission Technology).
 4. Themethod of claim 3, wherein initiating the scan comprises transmitting aScanning Interval Allocation Request (MOB_SCN-REQ) message.
 5. Themethod of claim 3, wherein detecting the trigger comprises: determiningthat a mean carrier-to-interference-plus-noise ratio (CINR) or a meanreceived signal strength indicator (RSSI) in the network service via thefirst RAT falls below a first threshold; and determining there is noother channel communicating via the first RAT with a mean CINR or a meanRSSI above a second threshold.
 6. The method of claim 3, whereindetecting the trigger comprises determining when a meancarrier-to-interference-plus-noise ratio (CINR) of all elements in adiversity set falls below a threshold.
 7. The method of claim 6, whereinthe threshold is equal to (1+γ)*H_Delete, γ is an adjustment factor ≧0,and H_Delete is a WiMAX threshold used to determine when to drop anelement from the diversity set.
 8. The method of claim 3, wherein theneighbor-listing message comprises a Neighbor Advertisement Message anddetecting the trigger comprises determining when the number of neighborsreceived in the Neighbor Advertisement Message (MOB_NBR-ADV) is lessthan β multiplied by an average number of the neighbors in previousNeighbor Advertisement Messages, wherein β is an adjustment factor ≧0.9. The method of claim 8, wherein the average number of the neighbors,represented as A_N_NBR(n) for the n^(th) Neighbor Advertisement Message,is an exponential moving average equal to α*N_NBR(n)+(1−α)*A_N_NBR(n−1),where α is a smoothing factor for the moving average and N_NBR(n) is thenumber of the neighbors for the n^(th) MOB_NBR-ADV message.
 10. Themethod of claim 1, wherein the first RAT is CDMA (Code Division MultipleAccess) EVDO (Evolution-Data Optimized) and the second RAT is WiMAX(Worldwide Interoperability for Microwave Access).
 11. The method ofclaim 10, wherein initiating the scan comprises transmitting “nullcover” as a Data Rate Control (DRC) cover.
 12. The method of claim 10,wherein detecting the trigger comprises determining when all pilots inthe CDMA active set have a pilot strength less than a threshold.
 13. Themethod of claim 12, wherein the threshold is equal to (1+γ)*T_DROP, γ isan adjustment factor ≧0, and T_DROP is a CDMA threshold used todetermine when to drop a pilot from the CDMA active set.
 14. The methodof claim 10, wherein the neighbor-listing message comprises a NeighborList Message and detecting the trigger comprises determining when thenumber of neighbors received in the Neighbor List Message is less than βmultiplied by an average number of the neighbors in previous NeighborList Messages, wherein β is an adjustment factor ≧0.
 15. The method ofclaim 14, wherein the average number of the neighbors, represented asA_N_NBR(n) for the n^(th) Neighbor List Message, is an exponentialmoving average equal to α*N_NBR(n)+(1−α)*A_N_NBR(n−1), where α is asmoothing factor for the moving average and N_NBR(n) is the number ofthe neighbors for the n^(th) Neighbor List Message.
 16. The method ofclaim 14, wherein the Neighbor List Message is at least one of anExtended Neighbor List Message, a General Neighbor List Message, or aUniversal Neighbor List Message.
 17. A receiver for wirelesscommunication, comprising: at least one processor configured to execute:trigger-detection logic configured to detect a trigger to initiate ascan for network service via a second radio access technology (RAT)while communicating via a first RAT, wherein the first and second RATsare different, wherein the trigger-detection logic is configured todetect the trigger when a number of neighbors received in aneighbor-listing message is less than a number of neighbors previouslyreceived in a neighbor-listing message; scan-initiation logic configuredto initiate the scan for the second RAT, in response to detecting thetrigger; and handover-determination logic configured to determinewhether to handover to network service via the second RAT based onresults of the scan; and a memory coupled to the at least one processor.18. The receiver of claim 17, wherein the trigger-detection logic isfurther configured to react to a periodic trigger.
 19. The receiver ofclaim 17, wherein the first RAT is WiMAX (Worldwide Interoperability forMicrowave Access) and the second RAT is CDMA (Code Division MultipleAccess) EVDO (Evolution-Data Optimized) or 1×RTT (one times RadioTransmission Technology).
 20. The receiver of claim 19, wherein thetrigger-detection logic is further configured to determine whether amean carrier-to-interference-plus-noise ratio (CINR) or a mean receivedsignal strength indicator (RSSI) in the network service via the firstRAT falls below a first threshold and to determine whether there is noother channel communicating via the first RAT with a mean CINR or a meanRSSI above a second threshold.
 21. The receiver of claim 19, wherein thetrigger-detection logic is further configured to determine when a meancarrier-to-interference-plus-noise ratio (CINR) of all elements in adiversity set falls below a threshold.
 22. The receiver of claim 21,wherein the threshold is equal to (1+γ)*H_Delete, γ is an adjustmentfactor ≧0, and H_Delete is a WiMAX threshold used to determine when todrop an element from the diversity set.
 23. The receiver of claim 19,wherein the neighbor-listing message comprises a Neighbor AdvertisementMessage and the trigger-detection logic is further configured todetermine when the number of neighbors received in the NeighborAdvertisement Message (MOB_NBR-ADV) is less than β multiplied by anaverage number of the neighbors in previous Neighbor AdvertisementMessages, wherein β is an adjustment factor ≧0.
 24. The receiver ofclaim 23, wherein the average number of the neighbors, represented asA_N_NBR(n) for the n^(th) Neighbor Advertisement Message, is anexponential moving average equal to α*N_NBR(n)+(1−α)*A_N_NBR(n−1), whereα is a smoothing factor for the moving average and N_NBR(n) is thenumber of the neighbors for the n^(th) MOB_NBR-ADV message.
 25. Thereceiver of claim 17, wherein the first RAT is CDMA (Code DivisionMultiple Access) EVDO (Evolution-Data Optimized) and the second RAT isWiMAX (Worldwide Interoperability for Microwave Access).
 26. Thereceiver of claim 25, wherein the trigger-detection logic is furtherconfigured to determine when all pilots in the CDMA active set have apilot strength less than a threshold.
 27. The receiver of claim 26,wherein the threshold is equal to (1+γ)*T_DROP, γ is an adjustmentfactor ≧0, and T_DROP is a CDMA threshold used to determine when to dropa pilot from the CDMA active set.
 28. The receiver of claim 25, whereinthe neighbor-listing message comprises a Neighbor List Message and thetrigger-detection logic is further configured to determine when thenumber of neighbors received in the Neighbor List Message is less than βmultiplied by an average number of the neighbors in previous NeighborList Messages, wherein β is an adjustment factor ≧0.
 29. The receiver ofclaim 28, wherein the average number of the neighbors, represented asA_N_NBR(n) for the n^(th) Neighbor List Message, is an exponentialmoving average equal to α*N_NBR(n)+(1−α)*A_N_NBR(n−1), where α is asmoothing factor for the moving average and N_NBR(n) is the number ofthe neighbors for the n^(th) Neighbor List Message.
 30. The receiver ofclaim 28, wherein the Neighbor List Message is at least one of anExtended Neighbor List Message, a General Neighbor List Message, or aUniversal Neighbor List Message.
 31. An apparatus for performinghandover between network service via first and second radio accesstechnologies (RATs), comprising: means for detecting a trigger toinitiate a scan for network service via the second RAT whilecommunicating via the first RAT, wherein the first and second RATs aredifferent, wherein the means for detecting a trigger is configured todetect the trigger when a number of neighbors received in aneighbor-listing message is less than a number of neighbors previouslyreceived in a neighbor-listing message; means for initiating the scanfor the second RAT, in response to detecting the trigger; and means fordetermining whether to handover to network service via the second RATbased on results of the scan.
 32. The apparatus of claim 31, wherein themeans for detecting the trigger is further configured to react to aperiodic trigger.
 33. The apparatus of claim 31, wherein the first RATis WiMAX (Worldwide Interoperability for Microwave Access) and thesecond RAT is CDMA (Code Division Multiple Access) EVDO (Evolution-DataOptimized) or 1×RTT (one times Radio Transmission Technology).
 34. Theapparatus of claim 33, wherein the means for detecting the trigger isfurther configured to determine whether a meancarrier-to-interference-plus-noise ratio (CINR) or a mean receivedsignal strength indicator (RSSI) in the network service via the firstRAT falls below a first threshold and to determine whether there is noother channel communicating via the first RAT with a mean CINR or a meanRSSI above a second threshold.
 35. The apparatus of claim 33, whereinthe means for detecting the trigger is further configured to determinewhen a mean carrier-to-interference-plus-noise ratio (CINR) of allelements in a diversity set falls below a threshold.
 36. The apparatusof claim 35, wherein the threshold is equal to (1+γ)*H_Delete, γ is anadjustment factor ≧0, and H_Delete is a WiMAX threshold used todetermine when to drop an element from the diversity set.
 37. Theapparatus of claim 33, wherein the neighbor-listing message comprises aNeighbor Advertisement Message and the means for detecting the triggeris configured to determine when the number of neighbors received in theNeighbor Advertisement Message (MOB_NBR-ADV) is less than β multipliedby an average number of the neighbors in previous Neighbor AdvertisementMessages, wherein β is an adjustment factor ≧0.
 38. The apparatus ofclaim 37, wherein the average number of the neighbors, represented asA_N_NBR(n) for the n^(th) Neighbor Advertisement Message, is anexponential moving average equal to α*N_NBR(n)+(1−α)*A_N_NBR(n−1), whereα is a smoothing factor for the moving average and N_NBR(n) is thenumber of the neighbors for the n^(th) MOB_NBR-ADV message.
 39. Theapparatus of claim 31, wherein the first RAT is CDMA (Code DivisionMultiple Access) EVDO (Evolution-Data Optimized) and the second RAT isWiMAX (Worldwide Interoperability for Microwave Access).
 40. Theapparatus of claim 39, wherein the means for detecting the trigger isfurther configured to determine when all pilots in the CDMA active sethave a pilot strength less than a threshold.
 41. The apparatus of claim40, wherein the threshold is equal to (1+γ)*T_DROP, γ is an adjustmentfactor ≧0, and T_DROP is a CDMA threshold used to determine when to dropa pilot from the CDMA active set.
 42. The apparatus of claim 39, whereinthe neighbor-listing message comprises a Neighbor List Message and themeans for detecting the trigger is configured to determine when thenumber of neighbors received in the Neighbor List Message is less than βmultiplied by an average number of the neighbors in previous NeighborList Messages, wherein β is an adjustment factor ≧0.
 43. The apparatusof claim 42, wherein the average number of the neighbors, represented asA_N_NBR(n) for the n^(th) Neighbor List Message, is an exponentialmoving average equal to α*N_NBR(n)+(1−α)*A_N_NBR(n−1), where α is asmoothing factor for the moving average and N_NBR(n) is the number ofthe neighbors for the n^(th) Neighbor List Message.
 44. The apparatus ofclaim 42, wherein the Neighbor List Message is at least one of anExtended Neighbor List Message, a General Neighbor List Message, or aUniversal Neighbor List Message.
 45. A mobile device, comprising: areceiver front-end for communicating via a first radio access technology(RAT); trigger-detection logic configured to detect a trigger toinitiate a scan for network service via a second RAT while communicatingvia the first RAT, wherein the first and second RATs are different,wherein the trigger-detection logic is configured to detect the triggerwhen a number of neighbors received in a neighbor-listing message isless than a number of neighbors previously received in aneighbor-listing message; scan-initiation logic configured to initiatethe scan for the second RAT, in response to detecting the trigger; andhandover-determination logic configured to determine whether to handoverto network service via the second RAT based on results of the scan. 46.The mobile device of claim 45, wherein the trigger-detection logic isfurther configured to react to a periodic trigger.
 47. The mobile deviceof claim 45, wherein the first RAT is WiMAX (Worldwide Interoperabilityfor Microwave Access) and the second RAT is CDMA (Code Division MultipleAccess) EVDO (Evolution-Data Optimized) or 1×RTT (one times RadioTransmission Technology).
 48. The mobile device of claim 47, wherein thetrigger-detection logic is further configured to determine whether amean carrier-to-interference-plus-noise ratio (CINR) or a mean receivedsignal strength indicator (RSSI) in the network service via the firstRAT falls below a first threshold and to determine whether there is noother channel communicating via the first RAT with a mean CINR or a meanRSSI above a second threshold.
 49. The mobile device of claim 47,wherein the trigger-detection logic is further configured to determinewhen a mean carrier-to-interference-plus-noise ratio (CINR) of allelements in a diversity set falls below a threshold.
 50. The mobiledevice of claim 49, wherein the threshold is equal to (1+γ)*H_Delete, γis an adjustment factor ≧0, and H_Delete is a WiMAX threshold used todetermine when to drop an element from the diversity set.
 51. The mobiledevice of claim 47, wherein the neighbor-listing message comprises aNeighbor Advertisement Message and the trigger-detection logic isconfigured to determine when the number of neighbors received in theNeighbor Advertisement Message (MOB_NBR-ADV) is less than β multipliedby an average number of the neighbors in previous Neighbor AdvertisementMessages, wherein β is an adjustment factor ≧0.
 52. The mobile device ofclaim 51, wherein the average number of the neighbors, represented asA_N_NBR(n) for the n^(th) Neighbor Advertisement Message, is anexponential moving average equal to α*N_NBR(n)+(1−α)*A_N_NBR(n−1), whereα is a smoothing factor for the moving average and N_NBR(n) is thenumber of the neighbors for the n^(th) MOB_NBR-ADV message.
 53. Themobile device of claim 45, wherein the first RAT is CDMA (Code DivisionMultiple Access) EVDO (Evolution-Data Optimized) and the second RAT isWiMAX (Worldwide Interoperability for Microwave Access).
 54. The mobiledevice of claim 53, wherein the trigger-detection logic is furtherconfigured to determine when all pilots in the CDMA active set have apilot strength less than a threshold.
 55. The mobile device of claim 54,wherein the threshold is equal to (1+γ)*T_DROP, γ is an adjustmentfactor ≧0, and T_DROP is a CDMA threshold used to determine when to dropa pilot from the CDMA active set.
 56. The mobile device of claim 53,wherein the neighbor-listing message comprises a Neighbor List Messageand the trigger-detection logic is configured to determine when thenumber of neighbors received in the Neighbor List Message is less than βmultiplied by an average number of the neighbors in previous NeighborList Messages, wherein β is an adjustment factor ≧0.
 57. The mobiledevice of claim 56, wherein the average number of the neighbors,represented as A_N_NBR(n) for the n^(th) Neighbor List Message, is anexponential moving average equal to α*N_NBR(n)+(1−α)*A_N_NBR(n−1), whereα is a smoothing factor for the moving average and N_NBR(n) is thenumber of the neighbors for the n^(th) Neighbor List Message.
 58. Themobile device of claim 56, wherein the Neighbor List Message is at leastone of an Extended Neighbor List Message, a General Neighbor ListMessage, or a Universal Neighbor List Message.
 59. A computer-programapparatus for performing handover between network service via first andsecond radio access technologies (RATs) comprising a non-transitorycomputer readable medium having instructions stored thereon, theinstructions being executable by one or more processors and theinstructions comprising: instructions for, while communicating via thefirst RAT, detecting a trigger to initiate a scan for network servicevia the second RAT, wherein the first and second RATs are different,wherein the trigger is detected when a number of neighbors received in aneighbor-listing message is less than a number of neighbors previouslyreceived in a neighbor-listing message; instructions for initiating thescan for the second RAT, in response to detecting the trigger; andinstructions for determining whether to handover to network service viathe second RAT based on results of the scan.
 60. The computer-programapparatus of claim 59, wherein the instructions for detecting thetrigger comprise instructions for reacting to a periodic trigger. 61.The computer-program apparatus of claim 59, wherein the first RAT isWiMAX (Worldwide Interoperability for Microwave Access) and the secondRAT is CDMA (Code Division Multiple Access) EVDO (Evolution-DataOptimized) or 1×RTT (one times Radio Transmission Technology).
 62. Thecomputer-program apparatus of claim 61, wherein the instructions fordetecting the trigger comprise: instructions for determining that a meancarrier-to-interference-plus-noise ratio (CINR) or a mean receivedsignal strength indicator (RSSI) in the network service via the firstRAT falls below a first threshold; and instructions for determiningthere is no other channel communicating via the first RAT with a meanCINR or a mean RSSI above a second threshold.
 63. The computer-programapparatus of claim 61, wherein the instructions for detecting thetrigger comprise instructions for determining when a meancarrier-to-interference-plus-noise ratio (CINR) of all elements in adiversity set falls below a threshold.
 64. The computer-programapparatus of claim 63, wherein the threshold is equal to (1+γ)*H_Delete,γ is an adjustment factor ≧0, and H_Delete is a WiMAX threshold used todetermine when to drop an element from the diversity set.
 65. Thecomputer-program apparatus of claim 61, wherein the neighbor-listingmessage comprises a Neighbor Advertisement Message and the instructionsfor detecting the trigger comprise instructions for determining when thenumber of neighbors received in the Neighbor Advertisement Message(MOB_NBR-ADV) is less than β multiplied by an average number of theneighbors in previous Neighbor Advertisement Messages, wherein β is anadjustment factor ≧0.
 66. The computer-program apparatus of claim 65,wherein the average number of the neighbors, represented as A_N_NBR(n)for the n^(th) Neighbor Advertisement Message, is an exponential movingaverage equal to α*N_NBR(n)+(1−α)*A_N_NBR(n−1), where α is a smoothingfactor for the moving average and N_NBR(n) is the number of theneighbors for the n^(th) MOB_NBR-ADV message.
 67. The computer-programapparatus of claim 59, wherein the first RAT is CDMA (Code DivisionMultiple Access) EVDO (Evolution-Data Optimized) and the second RAT isWiMAX (Worldwide Interoperability for Microwave Access).
 68. Thecomputer-program apparatus of claim 67, wherein the instructions fordetecting the trigger comprise instructions for determining when allpilots in the CDMA active set have a pilot strength less than athreshold.
 69. The computer-program apparatus of claim 68, wherein thethreshold is equal to (1+γ)*T_DROP, γ is an adjustment factor ≧0, andT_DROP is a CDMA threshold used to determine when to drop a pilot fromthe CDMA active set.
 70. The computer-program apparatus of claim 67,wherein the neighbor-listing message comprises a Neighbor List Messageand the instructions for detecting the trigger comprise instructions fordetermining when the number of neighbors received in the Neighbor ListMessage is less than β multiplied by an average number of the neighborsin previous Neighbor List Messages, wherein β is an adjustment factor≧0.
 71. The computer-program apparatus of claim 70, wherein the averagenumber of the neighbors, represented as A_N_NBR(n) for the n^(th)Neighbor List Message, is an exponential moving average equal toα*N_NBR(n)+(1−α)*A_N_NBR(n−1), where α is a smoothing factor for themoving average and N_NBR(n) is the number of the neighbors for then^(th) Neighbor List Message.
 72. The computer-program apparatus ofclaim 70, wherein the Neighbor List Message is at least one of anExtended Neighbor List Message, a General Neighbor List Message, or aUniversal Neighbor List Message.